Blizzard’s goal with layering is to provide many realms with 3k players in them, to simulate any given realm equivalent to launch, with minimal disappearances while you play. Its a worthy goal, when we expect a huge amount of drop off in the player base in the early days.
The problem they’re likely concerned with on Sharding only the 1-10 or even only 1-20, is that people can hit 20 within 1 day played easy, and so 20 may not be high enough before the population drops off again.
I totally think it will drop off after the hype dies down, but given what we’re seeing, people may get far further into the levelling process before they quit than I had originally predicted, and if Blizzard shares that concern, layering is there to hold them for that first week or two even if they get to 20-30 zones.
Adding protections to prevent the abuse of layer hopping, to me, seems like a far better solution to the problem than the usual “Throw it all out!” kneejerk reaction.